How to decrypt a dumped android partition on a pc?

I chose to boot android on the from the sdcard in order to have more storage. So I dumped though dd the internal 32Gb storage to the 256Gb sdcard with the partition table included. By just changing the kernel command line and the ro.boot.bootdevice property, the device now boots from the sd card after loading the kernel from the internal storage. * I then put the sd card in a pc, fixed the partition table size to match the one of the sd and then set up the userdata partition (/data) to fill the remaining space. * Then I wiped the filesystem and performed a factory reset in order to hope it would create a 250Gb filesystem for /data. But [while data was mounted from the sd card and that the mapped block device was 250Gb large](https://superuser.com/a/1537505/282033) , it was formated using 25Gb instead of 250GB (the original size on internal storage). This means I have to grow the /data filesystem using an external computer. Now the question is, what’s the exact dmsetup command in order to create the block device for decrypting it for growing the ext4 filesystem using resize2fs? Given also I forgot which is the file containing the decryption key.